Method of producing ferrite core assembly for magnetic storage devices



March 1, 1966 w. L. SHEVEL, JR., ETAL METHOD OF-PRODUCING FERRI'IE CORE ASSEMBLY FOR MAGNETIC STORAGE DEVICES Filed Dec. 13, 1961 FIG.I

INVENTORS' WILBERT L. SHEVEL, JR.

OTTO A. GUTWIN KURT R GREBE A TTORNE Y6 United States Patent 3,237,283 METHOD OF PRODUCING FERRITE CORE AS- SEMBLY FOR MAGNETIC STORAGE DEVICES Wilbert L. Shevel, Jr., Peekskill, Otto A. Gutwin, Crugers,

and Kurt R. Grebe, Beacon, N.Y., assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Dec. 13, 1961, Ser. No. 159,127 5 Claims. (Cl. 29-1555) This invention relates to an electromagnetic core assembly and to a process for making such an assembly. More specifically, the invention is directed to the production of a ferrile core assembly comprising a plurality of tubular ferrite components arranged in a coordinate array.

Magnetic core matrices are basic components of data processing apparatus. Their main function is to store digital data. Cores of the well known rectangular hysteresis loop type are basic components of such matrices. It is desirable that the core have optimum switching characteristics so that the operation of the matrix is both fast and reliable. Efforts to increase the switching-speeds of such cores have usually resulted in an increased cost of fabrication.

One important parameter to be considered in achieving a desired switching time is the cross-sectional area of the individual core members. The smaller the cross-sectional area of the core, the smaller is the total flux switched, but at the same time the cost of fabrication and the difficulty of threading and packaging such cores increases with decrease in size.

According to the prior art, ferrite core assemblies have been produced which incorporate a plurality of extruded ferrite core elements aligned and mounted on a support of non-magnetizable material. Even when using extruded, ring-shaped core elements of substantial cross-sectional area, the handling, aligning, mounting and threading of the elements is a difficult and expensive matter. Where the attempt is made to improve the performance of the assembly by utilizing cores of smaller cross-sectional area, the tasks of assemblying the array becomes increasingly tedious and costly.

In accordance with the present invention, the cross-sectional area of the cores may be materially reduced without a concurrent increase in the cost of fabrication or difliculty of handling. Thus, the production of a low-cost ferrite array which is easily handled and which has optimum switching characteristics is a major object of the present invention.

More specifically, it is an object of this invention to produce magnetic core matrices employing rectangular hysteresis loop cores of very small cross-sectional area without accompanying fabrication and handling difiiculties and expense.

It is a further object of this invention to produce matrices which are unique in structure, reliable in operation, easy to produce and handle and extremely fast with respect to switching time.

It will also be apparent that the present invention has for its object the production of a ferrite memory array in a manner which readily lends itself to continuous batch fabrication techniques with full use of automated devices, both in the production of the array and in subsequent wiring and handling.

The invention also has for its object the production of a ferrite memory array having high bit density, as well as the other desirable characteristics inherent in the cores of the extruded toroidal type.

The method for constructing ferrite core assemblies in accordance with the present invention broadly comprises the steps of extruding tubular members of ferrite material, assemblying overlaying tiers comprising a plurality of tube segments in the desired assembly and firing the assembly to produce an integral structure.

The foregoing and other objects, features and advantages of the invention will be apparent from the following description of the invention as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 is a perspective view of a partially assembled array in accordance with the invention;

FIG. 2 is a completed structure; and

FIG. 3 is a completed structure including threaded drive wires.

In preparing a ferrite core assembly according to the inevntion, the first general step is the extrusion of tubular ferrite core elements. This is accomplished by preparing ferrite powder, mixing the powder with a suitable binder to form an extrudable plastic mass and then die expressing this mass into tubular form. A plurality of tubular elements may be simultaneously extruded through a multiple die device or a single extrusion may be cut into lengths providing the required number of tubular elements.

The tubes are relative fragile when first extruded and so it is desirable, although not essential, to permit the green tubes to cure in air until their strength increases to the point at which they can be handled without danger of fracture or distortion.

Next, as shown in FIG. 1 of the drawing, a series of tubular segments are placed side by side with their longitudinal axes parallel to one another and with a predetermined distance between their respective aperture centers.

A second series or tier of parallel hollow ferrite tubes is then placed on top of the first tier of tubes, as shown in FIG. 2 of the drawing, making a single point of contact between the side surfaces of each tube in the first series and each tube in the second series. The longitudinal axes of the tubes in the second tier are also parallel to one another, but are set at an angle, for instance with respect to the longitudinal axes of the tubes in the lower tier.

The assembly is then fired or sintered in a furnace to produce intercrystalline growth .and bonding at the points of contact between the tubes in the first and second tiers. Upon cooling an integral structure is obtained which may be trimmed and threaded for use as a core assembly.

The invention including the foregoing general process and the products obtained according to the process will perhaps be better understood with reference to the following detailed example which constitutes a preferred embodiment of the invention and the best mode that has been contemplated for carrying it out.

In preparing the ferrite material for extrusion, a ferrite powder is first obtained and this is then mixed with an organic binder and low boiling solvents to produce the desired extrusion mixture.

In producing the ferrite powder, the following materials .are mixed in a ball mill for about four hours:

Fe O grams 924 MnCO do 980 Cr O do 34.68 NiO do 23.31 Methyl alcohol cc 4,000

After ball milling, the mixture is dried under a heat lamp and calcined at 850 C. for one hour in air. The calcined powder is then subjected to an additional ball milling to break up large sintered particles produced in the previous step and to generally reduce the particle size.

An additional 3,200 cc. of methyl alcohol is then added to the calcined powder and the mixture is ball milled for approximately forty-eight hours. The mixture is then screened through a 320 mesh screen and is dried under a heat lamp.

The ferrite powder produced accordingto the previous steps is then mixed with organic binder material and low boiling solvents in approximately the following proportions to produce a favorable consistency for extrusion:

Grams Ferrite powder (Fe Mn Cr NiQ from the preceding steps 20 Unfilled glyptal varnish 3 A-120 (7.5% ethyl cellulose and 92.5% pine oil) 1 Diethyl ether 4 The above mixture of ferrite powder, binder and low boiling solvents is then milled for fifteen minutes in a Spek Mixer Mill. A pug mill may also be used satisfactorily for this mixing step. During this mixing some of the solvents present may be evaporated, but this does not adversely affect the process or extruded products. When the fifteen-minute milling period has been completed, it is observed that the extrusion mixture has a tacky but not a sticky consistency and is characterized by the proper plastic flow in the extrusion device. Since the mix includes ether which evaporates rapidly, it is preferable to use it as soon as possible in order to maintain the proper solvent ratio. However, if the mix is cooled, it may be stored and used at a later time.

The ferrite extrusion mix is then loaded into the hopper of the extrusion device and hollow tubes of ferrite material are extruded through the die. A pressure of from ten to twenty-five pounds is maintained on the extrusion mix and an extrusion rate of from six to twelve inches per minute is also maintained.

The dimensions of the green extruded tubes are D. 0.025 inch, I.D. 0.015 inch and length approximately four inches. No support is necessary while the tubular memher is exiting from the extrusion press or during subsequent air curing provided that unreasonable lengths are not used.

The extruded hollow ferrite tubes are then air cured for approximately twenty-four hours in order to increase their green strength so that they may be handled without danger of breaking or dimensional alteration.

After the green tubes have been air cured, they are in a condition for assembly. A series or tier of the tubes in then arranged in a plane with their longitudinal axes parallel and with a predetermined distance between the axes.

Referring to FIG. 1 of the drawing, a series of three hollow ferrite tubes 10, 11 and 12 are shown in position at this stage of the assembly.

The spacing of the tubes, the number of the tubes, the physical dimensions of the tubes and the chemical composition of the ferrite material all are selected in accordance with the requirements of the final core assembly.

Next, cement is applied to the tubes as arranged in FIG. 1 of the drawing. It is immaterial whether the cement is applied to the tubes prior to or after their arrangement in this position.

In the preferred embodiment a ferrite cement having the following composition is applied in beads 20 along the upper surface of the tubes as shown in FIG. 1:

Grams Ferrite powder as prepared above 9 A-120 (7.5% ethyl cellulose and 92.5% pine oil) 3 Unfilled glyptal varnish 1 Pin Oil 1 Next, a second tier of hollow ferrite tubes 13, 14 and 15 is arranged on top of the first tier with their longitudinal axes in parallel alignment and with their axes spaced apart a predetermined distance. The longitudinal axes of the tubes in the second tier are set at an angle with respect to the longitudinal axes of the tubes in the first tier. The second tier of tubes is placed in contact with the beads of cement 20 applied tothe upper surfaces of the tubes 10, 11 and 12 of the lower tier. The cement 20 redissolves the skin of the tubes in the regions of contact and becomes an integral part of the tubes so as to form a preliminary bond between the tubes of the first and second tiers at their points of contact.

A cement-bonded assembly as shown in FIG. 2 of the drawing is produced by the foregoing step and is air cured to set the cement by heating in an oven at 120l50 C. for three hours. The cement may also be cured at room temperature, requiring a longer period of time to set.

As shown in FIG. 3, the angle between the tubes in the upper tier and the tubes in the lower tier is approximately but this is not critical. Other angular relationships may be worked out consistent with the desired characteristics of the particular assembly. Likewise, the number of tubes in each tier and the number of tier-s in the assembly is not limited to the arrangement shown in the drawing.

After the cement is cured, the assembly is fired, best results being obtained by placing the assembly in a furnace and gradually raising the temperature from ambient to 1,000 C. over a ten-hour period. During the treatment the binder material of the extrusion mix is volatilized and the structure is pre-sintered. Without cooling the assembly is then fired at 1,200" C. for approximately two hours. The assembly is cooled in the furnace to 950 C. and is then quenched in air.

In the preceding heating schedule, the rate of heating to 1000 C. is approximately C. per hour. The temperature is raised to 1200 C. as rapidly as the furnace will permit and the cooling rate, which is not critical, from 1200" C. to 950 C. is as fast as the furnace insulation will allow. For the particular ferrite mentioned, the lower desirable temperature is approximately 950 C. but this lower temperature will vary depending on the particular'ferrite which is being fired.

The firing schedule can be adjusted to suit the composition of the material being treated and the properties desired in the final product.

After the firing and sintering operations are completed, the tube ends may be trimmed, if needed, although this operation may be avoided by a preliminary trimming of the tubes while still green, i.e., before sintering.

The finished core assembly may then be wired as illustrated in FIG. 3. A plurality of drive lines X X X are threaded through the hollow cores 30 of lower tier tubes 10, 11 and 12 and a plurality of drive lines Y Y Y are threaded through hollow cores 40 of upper tier tubes 13, 14 and 15. The drive lines may be held in place in the tubes by any suitable means, such as insulating cemkent, a bracket inserted in the mouth of the tube, or the li e.

As a further variation, the assembly produced according to the present invention may be integrally mounted upon a non-magnetizable support plate. Such mounting is desirable in certain uses of the assembly. Several mounted or unsupported core assemblies may also be combined to form larger assemblies as needed.

The tubes have been shown in substantially parallel alingnment in the drawing. Although this is the most practical arrangement to produce orderly junctions and uniform spacing, arrays may also be produced with the tubes in non-parallel alignment.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that many changes in form and detail may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A process for producing a ferrite core assembly useful in memory arrays comprising forming a plurality of tubular ferrite elements, placing a series of said elements in spaced apart, parallel alignment, placing a second series of said elements in spaced apart, parallel alignment and in contact with said first series of elements so that each element of said first series has a single point of contact with each element of said second series, sintering said first and second series while in contact to form an intercrystalline ferrite between the tubes of said first series and said second series at each point of contact and to produce a self-supporting sintered ferrite core assembly.

2. A process for producing a ferrite core assembly for use in memory devices comprising forming a plurality of tubular ferrite elements, placing a series of at least two of said elements in parallel alignment, placing a second series of said elements containing the same number of elements as in said first series in parallel alignment and in contact with said first series so that each element of said first series has a point of contact with each element of said second series, sintering said first and second series of elements while in contact to bond said first series to said second series at each of said points of contact producing a self-supporting structure and threading wire conductors through the cores of said tubular elements to produce the finished core assembly.

3. A process for producing a ferrite core assembly useful in memory devices comprising mixing ferrite powder with a volatilizable organic binder to produce an extrudable, plastic mass, extruding a plurality of tubular ferrite core elements from said plastic mass, curing the freshly extruded elements to increase their strength, placing a series of at least two of said elements in parellel alignment, cementing to the elements of said first series a second series of the same number of said elements also in parallel alignment so that each element of said first series has a cemented bond with each element of said second series producing a cement-bonded assembly, curing said cement-bonded assembly, sintering said cementbonded assembly to produce intercrystalline bonding between the elements of said first and second series, to drive off the residual organic binder and to sinter the ferrite material and finally threading wire conductors through the cores of said tubular elements to produce the desired ferrite core assembly.

4. A process for producing a ferrite core assembly useful in memory devices comprising mixing ferrite powder with a volatilizable organic binder to produce an extrudable, plastic mass, extruding a plurality of tubular ferrite core elements from said plastic mass, curing the freshly extruded elements to increase their strength, placing a series of at least two of said elements in parallel alignment, cementing to the elements of said first series a second series of the same number of said elements also in parallel alignment so that each element of said first series has a cemented bond with each element of said second series producing a cement-bonded assembly, curing said cement-bonded assembly, sintering said cement-bonded assembly by heating to 1,000 C. to volatilize remaining organic binder and to effect a preliminary sinter of the assembly and then heating to 1,200 C., to effect a final sintering of the assembly.

5. The process for producing a ferrite core assembly comprising mixing powdered ferrite material with a volatilizable organic binder to form a homogeneous extrudable plastic mass, extruding a plurality of self-supporting, tubular, ferrite core elements from said plastic mass, placing a series of said elements in parallel alignment, placing beads of a ferrite cement at selected points on the surface of each element of said first series, placing a second series of said elements in contact with the elements of said first series also in parallel alignment, said contact being at said selected points, curing said ferrite cement to produce a cement-bonded assembly of said first and second series, and sintering said cement-bonded assembly to drive off said volatilizable organic binder and to produce intercrystalline bonds between the elements of said first and second series at said points of contact.

References Cited by the Examiner UNITED STATES PATENTS 2,385,386 9/1945 Stolfel.

2,724,103 11/ 195 5 Ashenhurst.

2,725,265 11/1955 Daniels et al. 208 X 2,771,969 11/1956 Brownlow 29472.9 X 2,828,981 4/ 1958 Reed.

2,910,673 10/1959 Bloch 340-174 2,934,748 4/ 1960 Steimen 340-174 2,961,745 11/1960 Smith 29155.5 2,970,905 2/ 1961 D011 75208 2,985,948 5/1961 Peters 29155.5 2,993,111 7/ 1961 Schrewelius et al. 29-472.9 X 3,020,632 2/ 1962 Krikorian et al. 29420 3,077,021 2/1963 Brownlow 29-l55.5 X 3,100,295 8/ 1963 Schweizerhof 29155.5

WHITMORE A. WILTZ, Primary Examiner.

IRVING L. SRAGOW, JOHN F. CAMPBELL,

Examiners. 

1. A PROCESS FOR PRODUING A FERRITE CORE ASSEMBLY USEFUL IN MEMORY ARRAYS COMPRISING FORMING A PLURALITY OF TUBULAR FERRITE ELEMENTS, PLACING A SERIES OF SAID ELEMENTS IN SPACED APART, PARALLEL ALIGNMENT, PLACING A SECOND SERIES OF SAID ELEMENTS IN SPACED APART, PARALLEL ALIGNMENT AND IN CONTACT WITH SAID FIRST SERIES OF ELEMENTS SO THAT EACH ELEMENT OF SAID FIRST SERIES HAS A SINGLE POINT OF CONTACT WITH EACH ELEMENT OF SAID SECOND SERIES, SINTERING SAID FIRST AND SECOND SERIES WHILE IN CONTACT TO FORM AN INTERCRYSTALLINE FERRITE BETWEEN THE TUBES OF SAID FIRST SERIES AND SAID SECOND SERIES AT EACH POINT OF CONTACT AND TO PRODUCE A SELF-SUPPORTING SINTERED FERRITE CORE ASSEMBLY. 